1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a facsimile, or a copying machine, using an electrophotographic process or an electrostatic recording process.
2. Description of the Related Art
A variety of image forming apparatuses using an electrophotographic process or an electrostatic recording process as an image forming process have been developed. An example of such a variety of image forming apparatuses is an intermediate transfer tandem type image forming apparatus having a plurality of process cartridges arranged in a line along a rotational direction of an intermediate transfer belt rotatably tensioned to form a color image via an intermediate transfer belt.
In the image forming apparatus, each of the process cartridges includes a photosensitive drum. A primary transfer roller is disposed so as to face each of the photosensitive drums with the intermediate transfer belt therebetween. The primary transfer rollers correspond to a yellow (Y) color, a magenta (M) color, a cyan (C) color, and a black (Bk) color. The timings at which transfer operations performed by the primary transfer rollers Y, M, C, and K (corresponding to the colors) are shifted sequentially from each other. The timings of switching ON-OFF of transfer bias applied to the primary transfer rollers are shifted sequentially in synchronization with the timings at which the transfer operations are performed.
The ON-OFF switching of the transfer bias applied to the primary transfer rollers causes a variation of the moving speed of the intermediate transfer belt, causes a variation of the position in the sub-scanning direction and, thus, causes out of color registration. That is, if transition from the transfer-on state to the transfer-off state occurs, transition from a state in which the intermediate transfer belt is electrostatically attracted to the photosensitive drum to a state in which the intermediate transfer belt is not electrostatically attracted to the photosensitive drum occurs. Accordingly, a variation of the speed of the intermediate transfer belt occurs due to a change in the attraction state between the intermediate transfer belt and the photosensitive drum. Thus, an “out of color registration” problem arises. In addition, in the image forming apparatus, when the downstream primary transfer roller performs a transfer operation, the timings at which the transfer bias is turned off for the upstream primary transfer roller is the same for all primary transfer units. Thus, a variation of the position occurs at the same position of a recording medium. As a result, a line-like image artifact that extends on the recording medium (a sheet) in the main scanning direction may become prominent. That is, a line-like image may disadvantageously become darker.
Japanese Patent Laid-Open No. 2005-148198 describes an image forming apparatus to address such an issue. In the image forming apparatus, when an overlapped image formed on the intermediate transfer belt is secondary-transferred onto a recording medium, the transfer bias is continuously applied to all the primary transfer rollers for a period of time from the time the transfer operation performed by the Y primary transfer roller starts until the transfer operation performed by the Bk primary transfer roller is completed. By avoiding ON-OFF switching of application of transfer bias to any one of the primary transfer rollers in this manner, a variation of the speed of the intermediate transfer belt caused by ON-OFF switching is prevented and, thus, degradation of the image quality, such as out of color registration, can be reduced.